Excess iron has been strongly implicated in many diseases including diabetes and nonalcoholic steatohepatitis (NASH). At the other extreme, iron deficiency, the most common nutritional disorder in the world, contributes to obesity. Understanding the effects of iron on metabolism has important implications for all of these conditions. First, iron may play a fundamental, causal role in their pathogenesis, as has been demonstrated by interventional studies in human diabetes, NASH, and to a lesser degree vascular disease. This would support treating the disorders by manipulating iron stores (e.g. by blood donation); we need to understand the full ramifications of doing so to establish rational treatment goals of optimal iron. Second, understanding mechanisms underlying iron's effects will open doors to other druggable pathways. The need to define the effects of iron and their dose-responsiveness is illustrated by the effects on hepatic lipid metabolism. High iron is an established risk factor for the progression of nonalcoholic fatty liver disease (NAFLD) to NASH, but iron deficiency is a risk factor for NAFLD and obesity. Thus, there should be an existing intermediate level of iron that fully supports fuel oxidation and does not contribute to obesity bu that is also not in the range to contribute to the hepatic inflammation and scarring of NASH. Understanding the dose-responsiveness for the metabolic effects of iron is especially needed given the very wide range of normal iron, which is ~15-fold. In our previous work, we have demonstrated in mice that dietary iron, within the range of normal diets and without overt iron deficiency or iron toxicity, has major effects on carbohydrate metabolism. Preliminary data demonstrate that iron also regulates key aspects of lipid metabolism including hepatic de novo lipogenesis, fatty acid (FA) oxidation, and leptin synthesis. This proposal is to study the effects of iron on these processes, with the aims of describing a fuller phenotype of lipid metabolism as a function of tissue iron and of determining the mechanism for these effects. Our overall hypothesis is that iron levels can contribute oth to NAFLD, NASH, and obesity, and that manipulating these over a much narrower range than the normal one of the US population will have significant health implications.